1. Field of the Invention
The present invention relates to an amplitude modulation detector, an apparatus and method for detecting a CAS (Customer premise equipment Alert Signal), and an apparatus and method for detecting caller identification.
2. Description of the Related Art
In general, a caller ID system called xe2x80x9ccaller identification servicexe2x80x9d is a system for transferring information to customers through telephone lines. The information may include caller information such as a caller""s telephone number or a caller""s name, or general information regarding topics such as the weather or traffic. However, the use of telephone lines for voice communications may limit the transfer of an information signal. For example, in transferring a caller ID through a telephone line, identifying a CAS lying in the same frequency band as a voice signal can be difficult. Thus, telephone operation includes a general talk mode and an information reception mode, and a signal applied only in the information reception mode can be more easily identified as a CAS.
For the caller identification service, a CAS is used to switch between the general talk mode and the information reception mode. The CAS is a dual tone signal in which two signals with different frequency values, e.g., 2.13 KHz and 2.75 KHz, are mixed according to the system standard. When a transmitting site, i.e., a central office, intends to transmit information in the general talk mode, the transmitting site sends a CAS through a telephone line to a receiving site, before transmission of the information. Here, a CID/CIDCW (Calling Identity Delivery/Calling Identity Delivery on Call Waiting) receiver at the receiving site transmits to the central office a CAS acknowledgment signal that indicates detection of the CAS. The CID/CIDCW receiver can display the caller ID on customer premise equipment (CPE) in a network. The CID/CIDCW receiver is in the CPE. Here, the receiving site transmits the CAS acknowledge signal to the central office, when changing the general talk mode into the information receiving mode.
If a receiver is in an OFF-HOOK state, i.e., a user is calling another person, a voice signal of the user may be mixed with the CAS. Accordingly, the receiving site must detect the CAS from a mixed signal including the CAS and the voice signal.
FIG. 1 is a block diagram of a conventional CAS detection apparatus. The conventional CAS detection apparatus includes a first band-pass filter (BPF) 10, a second BPF 12, a first frequency discriminator 14, a second frequency discriminator 16, and a CAS discriminator 18. BPF 10 filters an input signal IN and passes signal components in a first frequency band, i.e., the 2.13-KHz band. BPF 10 passes signal components in a second frequency band, i.e., the 2.75-KHz band, from the input signal IN.
Frequency discriminators 14 and 16 receive the signals from BPFs 10 and 12, respectively, and determine whether the signals have respective frequencies 2.13 KHz and 2.75 KHz, which correspond to a CAS. If a CAS is in the input signal, the filtered signals from BPFs 10 and 12 have significant amplitudes for the frequency components having frequencies 2.13 KHz and 2.75 KHz. However, if the CAS is not present in the input signal and a voice signal is presents, the filtered signals typically have frequencies other than 2.13 KHz and 2.75 KHz or small amplitudes at frequencies 2.13 KHz and 2.75 KHz. CAS discriminator 18 receives the output signals of frequency discriminators 14 and 16, and generates a CAS detection signal CAS_DET if signals with the frequencies 2.13 KHz and 2.75 KHz are present.
The conventional CAS detection apparatus of FIG. 1 utilizes multiple paths to detect multiple tones and identify the presence of the CAS. The multiple paths increase circuit complexity. Also, if the voice portion of the input signal includes frequency components of frequencies 2.13 KHz and 2.75 KHz, the CAS detection apparatus may generate a false CAS detection.
FIG. 2 is a block diagram of another conventional CAS detection apparatus. The conventional CAS detection apparatus of FIG. 2 includes a first signal detector 100, a second signal detector 150, and a timing selector 180.
The first signal detector 100 detects a first frequency component of a CAS in input signal IN. The second signal detector 150 detects a second frequency component of the CAS. Here, detectors 100 and 150 indicate the presence of the first and second frequency components to timing selector 180. If the first and second frequency components remain for a predetermined time, timing selector 180 determines that the input signal contains a CAS and outputs a CAS detection signal CAS_DET.
In signal detector 100, a guard band filter 102 passes the first frequency component filtered from the input signal. A first AC-DC converter 112 converts the first frequency component to a DC signal having a voltage proportional to the amplitude of the first frequency component. A first threshold detector 114 detects whether the DC signal from AC-DC converter 112 is at a voltage higher than a first threshold value. To distinguish noise in the input signal IN, AC-DC converter 112 and threshold detector 114 preferably operate first to provide a control signal to a limiter 104 or a post limiter filter 106. If the voltage of the DC signal is above the first threshold value, limiter 104 amplifies the frequency component signal from the guard band filter 102 to a predetermined level. Post limiter filter 106 filters the amplified signal before another AC-DC converter 108 converts the amplified frequency component signal to another DC signal. A threshold detector 110 compares the voltage of the DC signal from converter 108 to a second threshold value. If the DC signal has a voltage higher than the second threshold value, the second threshold detector 110 sends to the timing selector 180, a signal indicating the presence of the first frequency component in input signal IN. A process detector 150 uses for detecting a second frequency signal in input signal IN is similar to the above. Timing selector 180 detects whether the detected first and second frequency signals remain for a predetermined time and asserts a CAS detection signal CAS_DET if they do.
One drawback of the conventional CAS detection apparatus of FIG. 2 is the use the analog circuitry to detect the frequencies and the levels of the input signal. The required analog circuitry is difficult to form in an integrate circuit. For example, the CAS detection apparatus of FIG. 2 requires external components such as a capacitor, for detecting the frequency and the level of the CAS. The capacitor must be external to an integrated circuit because the required capacitance for the CAS detection apparatus is large. Thus, the CAS detection apparatus is difficult to implement in a single chip.
FIG. 3 is a block diagram of still another conventional CAS detection apparatus. The conventional CAS detection apparatus of FIG. 3 includes a high tone filter 210, a low tone filter 220, first and second level comparators 215 and 225, a tone detector 240, and a guard time detector 280.
High tone filter 210 passes only the high frequency band of the CAS from input signal IN. Then, level comparator 215 compares the filtered signal with a first internal reference and applies a signal indicating the result of the comparison to the tone detector 240. Similarly, low tone filter 220 passes only the low frequency band of the CAS from input signal IN. Level comparator 225 compares the filtered signal with a second internal reference and applies a signal indicating the result of that comparison to tone detector 240. Tone detector 240 receives the result signals from level comparators 215 and 225 and determines whether each level is higher than a predetermined reference level. If the output signals of level comparators 215 and 225 are higher than the predetermined reference level, tone detector 240 determines that input signal IN contains a CAS. As described above, high tone filter 210 and low tone filter 220 detect the frequencies of the CAS, and level comparators 215 and 225 detect the levels of the CAS. If input signal IN is identified as containing a CAS, guard time detector 180 detects whether the CAS maintains for the required levels for a predetermined time and generates CAS detection signal CAS_DET if the CAS remains for the predetermined time.
In guard time detector 280, a NAND gate 282 performs a NAND operation on a tone detection signal Est output from tone detector 240 and the output signal of a comparator 285. NAND gate 282 applies a result signal to the gate of a PMOS transistor MP1. Also, a NOR gate 287 performs a NOR operation on tone detection signal Est from tone detector 240 and the output signal of comparator 285, and applies a result signal to the gate of an NMOS transistor MN1. Here, comparator 285 compares a threshold voltage VTRH to the drain voltage of PMOS transistor MP1 and NMOS transistor N1. Comparator 285 outputs signal CAS_DET at a high level, indicating detection of a CAS, when the drain voltage is higher than the threshold voltage VTRH. For detecting the sustain time of the CAS, an external capacitor C1 and an external resistor R1 have a time constant that controls charging of the drain voltage so that the drain voltage only reaches the threshold voltage VTRH if signal Est remains asserted for the required time for the CAS. Accordingly, the CAS detection apparatus of FIG. 3 also requires an external component such as a capacitor that is difficult to implement in a single-chip CAS detection apparatus.
Similarly, a CID/CIDCW receiver of a receiving site detects a CAS and transmits a CAS acknowledge signal to acknowledge the CAS. The central office that receives the CAS acknowledge signal from the receiving site transmits information about the caller to the CID/CIDCW receiver at the receiving site. After the reception of information is completed, the receiving site changes its mode to the general talk mode. Here, the information about the caller transmitted from the central office is frequency shift keyed (FSK) data. The CID/CIDCW receivers of the CPE demodulates an FSK signal to detect the original information about the caller and displays the detected information in the CPE. Thus, the conventional apparatus for detecting information about a caller requires circuitry to detect a CAS and external components that are difficult to implement in a single-chip integrated circuit.
In accordance with an aspect of the invention, a CAS (Customer premise equipment Alert Signal) detection apparatus minimizes the number of paths required to detect tones of a CAS and detects the CAS without false detection due to a voice signal. Further, the CAS detection apparatus can be implemented in a single chip without needing external components.
A further aspect of the invention provides a novel amplitude modulation detector used in the CAS detection apparatus and in an apparatus detecting caller information.
An embodiment of the present invention further provides an apparatus for detecting information about a caller where use of a single chip without an external component reduces the size of the apparatus.
In accordance with one embodiment of the invention, a CAS detector includes a mixing unit, an amplitude modulation detection unit, and a frequency discriminating unit. The mixing unit mixes two or more frequency components filtered from the input signal to generate an amplitude modulated signal having a carrier and an envelope. The amplitude modulation detection unit measures the amplitude modulation, and the frequency discriminating unit detects whether the frequency of the envelope is in a predetermined range indicating presence of a CAS.
In accordance with another embodiment of the invention, a CAS detection apparatus for detecting a CAS from an input signal, includes a mixing unit, an amplitude modulation detection unit, a signal detection unit, and a CAS discrimination unit. The mixing unit mixes two or more frequency components filtered the input signal to generate an amplitude modulated signal having a carrier and an envelope. The amplitude modulation detection unit measures the amplitude modulation. The signal detection unit determines whether the frequency of the detected envelope is in a predetermined range indicating presence of the CAS and compares a voltage level of the envelope to a reference voltage to determine whether the level is characteristic of the CAS. The CAS discrimination unit receives the results from the signal detection unit and determines whether or not the CAS is present.
In accordance with still another embodiment of the invention, a CAS detection apparatus includes a mixing unit, a signal conversion unit, an amplitude modulation detection unit, a signal detection unit, and a CAS discrimination unit. The mixing unit mixes two or more frequency components filtered from an input signal to generate an amplitude modulated signal having a carrier and an envelope. The signal conversion unit automatically adjusts the gain of an output signal from the mixing unit to maintain the output signal in a range between a first threshold voltage and a second threshold voltage and converts the gain adjusted signal into a digital signal. The amplitude modulation detection unit analyzes the output signal from the signal conversion unit to identify the envelope. The signal detection unit determines whether the frequency of the envelope is in a range corresponding to the CAS and detects the difference between the maximum and minimum levels of the envelope to determine whether the component levels are characteristic of the CAS. The CAS discrimination unit receives the results of discriminating the frequency and voltage level by the signal detection unit and determines whether or not the CAS is present.
In accordance with another embodiment of the invention, an amplitude modulation detector for a CAS detection apparatus detects an amplitude modulated signal including a carrier and an envelope. The amplitude modulation detector includes a carrier peak detection unit and an envelope level detection unit. The carrier peak detection unit receives and shifts digital samples of the amplitude modulated signal in response to a clock signal and compares sets of digital samples to identify peaks of the carrier. The envelope level detection unit receives the digital samples for the identified carrier peaks in response to a second clock signal, and compares sets of peaks to identify extrema (i.e., maxima and/or minima) of the envelope.
In accordance with yet another embodiment of the invention, a CAS detection apparatus includes a mixing unit, a signal conversion unit, an amplitude modulation detection unit, a signal prediction unit, a signal detection unit, a CAS discrimination unit. The mixing unit filters the input signal into two or more frequency components, and mixes filtering results to generate an amplitude modulated signal including a carrier and an envelope. The signal conversion unit automatically controls a gain of an output signal of the mixing unit so that the output signal is in a first range between a first threshold voltage and a second threshold voltage, and converts the gain adjusted signal into a digital signal. The amplitude modulation detection unit detects the peaks of the carrier and envelope from the digital signal output from the signal conversion unit. The signal prediction unit detects a level of the amplitude modulated signal and occurrence periods and the number of peaks in the carrier and envelope to predict the presence of the CAS in the input signal, and outputs the prediction result as a CAS prediction signal. The signal detection unit determines whether a frequency of the envelope detected by the amplitude modulation detection unit is in a second range corresponding to the CAS, and detects a maximum/minimum level difference of the envelope to determine whether the maximum/minimum level corresponding to the CAS. The CAS discrimination unit determines whether the detection results from the signal detection unit remains for a predetermined period to determine whether the CAS is present.
In accordance with another embodiment of the invention, a CAS detection method includes: selecting two or more frequency components of an input signal and mixing the frequency components to generate an amplitude modulated signal; measuring an envelope of the amplitude modulated signal; and discriminating whether the frequency and voltage level of the envelope are in a range corresponding to the presence of the CAS.
Another CAS detection method includes: selecting two or more frequency components of an input signal and mixing the frequency components to generate an amplitude modulated signal; measuring an envelope of the amplitude modulated signal; predicting that the CAS is present if a carrier peak remains higher than a predetermined level for a predetermined period; discriminating whether the frequency and voltage level of the envelope are in a predetermined range if it is predicted that the CAS is present; and determining that the CAS is present if the frequency and level of the envelope satisfy the conditions.
Still another CAS detection method includes: filtering the input signal into two or more frequency components and mixing filtering results to generate an amplitude modulated signal; measuring a carrier and an envelope of the amplitude modulated signal; removing error peaks from peaks of the carrier and envelope, and predicting that the CAS is present if peaks in the carrier and envelope remain higher than a predetermined level for a predetermined period; if it is predicted that the CAS is present, determining whether a frequency and a voltage level of the envelope is in a predetermined range using the peaks of envelope from which the error peaks have been removed; and if it is determined that the frequency and the voltage level of the envelope is in the predetermined level, detecting a duration of the CAS to determine whether the CAS is present.
In accordance with another embodiment of the invention, a caller information detection includes a signal conversion unit, a peak detector, and a frequency shift keying (FSK) demodulation unit. The signal conversion unit filters the input signal to select two or more frequency components and converts the results into a digital signal. The peak detector detects peaks of the digital signal and assets a peak detection signal upon detecting a maximum or minimum in the digital signal. The frequency and number of maxima and minima indicate a MARK or SPACE signal and associated bit values. The FSK demodulation unit discriminates the occurrence number and interval of the maxima and minima to identify the MARK or SPACE signal and generate FSK data, detects the edge of the FSK data signal to generate clock signals for storing or transferring the FSK data.
In accordance with another embodiment of the invention, a caller information detection method includes: selecting two or more frequency components of an input signal and converting the results into a digital signal; detecting maxima and minima of the digital signal to identify a MARK signal or a SPACE signal and generate frequency shift keyed (FSK) data; detecting the edge of the FSK data signal to generate an FSK clock signal; generating a data clock signal in response to the FSK clock signal and the FSK data, the data clock signal being enabled in only an actual data interval; and transferring the FSK data in response to the data clock signal.